(1) Field of the Invention
The present invention relates to a method for surface treating conductive copper powder for use in conductive pastes such as conductive inks, conductive coatings and conductive adhesives which possess excellent conductivity and environmental adaptability.
(2) Description of the Prior Art
As is known, it has long been considered to use copper powder as conductive filler in conductive paste. However, the metal copper powder, particularly the fine copper powder, can be oxidized rapidly in the air. Therefore, to prevent copper powder from oxidization is a technical problem in the development of conductive pastes. To improve oxidation resistance of copper powder so as to enable it to be used in conductive paste, various proposals, including methods of surface treating the copper powder, methods of employing additives and combinations thereof, have been made.
In the method of surface treating, the copper powder is treated with an antioxidant or coated with a metal such as tin, nickel, platinum or silver. Japanese Unexamined Patent Publication No. 113505/1982(JP57-113505A) discloses a conductive coating composition in which a tin-coated copper powder is employed as conductive filler. JP60-100679A and JP60-243277A disclose methods for coating copper powder with silver.
In the method of employing additives, reducing agent and/or coupling agent are incorporated as essential components into the conductive paste when the copper powder is dispersed in a solution of synthetic resin binder. The conductive paste thus obtained exhibits oxidation resistance to a certain extent.
In the combination method, metal-coated copper powder is treated with a coupling agent.
Examples of the materials to be used as antioxidant that may be mentioned are amines, organo-phosphorous, organo-silane, organo-aluminium and organo-titanium compounds. In U.S. Pat. No. 4,663,079, phosphites are employed; in JP61-200179A, higher fatty amines are used; in U.S. Pat. Nos. 4,305,847 and 4,382,981 and JP58-74579A and 59-75931A, 59-174661A, organo-titanium compounds are used; in JP59-179671A and JP60-3200A, organic aluminates or titanates are used; in JP62-280271A, silanes and titanates are employed.
Examples of the additives used are phosphoric acid, phosphorous acid and esters thereof, phenols and derivatives thereof, aldehydes, hydrazines, ascorbic acid, triazole derivatives, imidazole derivatives, reducing sugar and coupling agents. JP60-258273A discloses a conductive coating composition in which the copper powder employed is treated with citric acid or tartaric acid; JP57-55974A and 96401A disclose conductive coating compositions in which hydroxyphenol derivatives such as hydroquinone, catechol are added; U.S. Pat. No. 4,705,647 discloses a conductive composition in which triazole derivatives are used as additives; JP60-243171A discloses a conductive composition in which organic carboxylic acid salts of alkyl imidazoles, nitrogen- or sulfur-containing silane couplers, phosphates and/or phosphites are added.
The conductive pastes obtained according to the above-mentioned first and second methods have good initial electrical conductivity, but they have poor storage stability and environmental adaptability. The conductive pastes obtained according to the third method have better properties, but are still not ideal. In brief, of the conductive pastes afore-mentioned, some have the drawback of forming patina, some have the deficiency of aggregation and sedimentation of fillers, some change greatly in conductivity after a period of time and some do not exhibit sufficient degree of heat resistance and humidity resistance.
U.S. Pat. No. 4,781,980 discloses a method for treating copper powder on the surface of which an antioxidation film of a salt of an aliphatic mono- or di-amine, having aliphatic hydrocarbon having 12 to 22 carbon atoms, of a mono- or di-carboxylic acid, having either aliphatic hydrocarbon having 2 to 23 carbon atoms or aromatic hydrocarbon groups having 7 to 8 carbon atoms, is formed; and a film comprising a dispersing agent of the type of boron-nitrogen composite and at least one coupler selected from the group consisting of titanate and aluminate is then formed on the antioxidation film. The boron-nitrogen composite type dispersing agent is a dispersing agent in which a boron-containing surfactant substance and a nitrogen-containing surfactant substance are blended and combined each other. The boron-containing surfactant substance has in the molecule thereof a boronic acid ester bond with polyhydric alcohol, and has one or more groups of hydrocarbons having 7 to 25 carbon atoms. The nitrogen-containing surfactant substance has one or more groups of hydrocarbons having 7 to 25 carbon atoms, and has non-covalent electron pairs. The copper powder thus-obtained is capable of maintaining stable conductivity for a long period of time, and possesses a good affinity for synthetic resin and a high degree of thermal resistance.
JP02-18469A discloses a copper powder for conductive coating and conductive coating composition using same. The copper powder is coated with a zirconate compound and a hydroxyalkylamine, together with a higher fatty acid ester. The obtained copper powder thus obtained has rust proofing properties and capable of improving conductivity and environmental adaptability by adding to a coating.
As is known to the persons skilled in the art, conductive copper paste should possess excellent conductivity and generally has a volume resistance of 10.sup.-3 to 10.sup.-4 .OMEGA..multidot.cm. In order to ensure this conductivity, copper powder should have good oxidation resistance. For obtaining pure copper powder, a known method is employed which comprises removing the organic protective layer (fatty acids and other organics) on the surface of the copper powder and then removing the oxides thereon with an acid.
In addition, whether the copper powder is well dispersed in binder resins would obviously affect the conductivity of the conductive copper paste. Furthermore, the storage stability and the environmental adaptability of the conductive paste are important quality indices for practical applications of the conductive paste. In general, the change of the conductivity of a conductive paste should be less than 10% . Troubles would be found in application if the limit is exceeded.